By Josh Davy, Guy Kyser, Joseph DiTomaso, Matthew Rinella, and Grace Woodmansee 1
Edited by Celestine Duncan
Medusahead (Taeniatherum caput-medusae) is an invasive annual winter grass that plagues western rangelands in eight western states (Figure 1). Controlling this grass is a challenge, in part due to difficulty in selectively targeting an undesired single grass in a grassland environment and because the economic return on investment for control can be low on western rangelands. Previous research showed that medusahead could be controlled by applying Milestone® herbicide pre-germination in the fall at 14 fluid ounces per acre (fl oz/A), with suppression achieved at 7 fl oz/A. However, the 14 fl oz/A rate is only registered for use as a spot treatment with not more than 50 percent of an acre treated.
Greenhouse studies by Dr. Matt Rinella with USDA-ARS in Montana found a reduction in medusahead’s ability to produce seed if Milestone was applied during the boot growth stage. The boot stage occurs just prior to seedhead emergence (Figure 2). Since most medusahead seeds germinate within a year of production, any treatment that prevents seed production can drastically reduce the weed population.
Field trials were established in Northern California to test the efficacy of Milestone® herbicide applied at 3, 7 and 14 fl oz/A in reducing medusahead seed production when applied at the boot growth stage. At the Red Bluff site, seed germination tests found that all rates of Milestone were equal in drastically reducing medusahead seed production from over 80 percent viable seeds to less than 10 percent. It is important to note that Milestone did not kill medusahead, but rather inhibited the plant from producing viable seed.
Medusahead cover data were collected the following season and results corresponded to reduction in viable seed production (Table 1). Milestone treatments significantly reduced medusahead cover compared to non-treated plots. There was no significant difference between 3, 7 or 14 fluid ounces of Milestone; however, at the Red Bluff site there was a trend toward greater reduction in medusahead cover with increasing herbicide rate.
Proper application timing of Milestone is critical to reduce medusahead viable seed production. Treatments must be made to medusahead at the boot growth stage to reduce populations the following year. The boot stage of plant growth occurs just before seedhead emergence (heading). Far lower success in medusahead seed suppression would occur if application timing occurred at or after heading.
The higher rates of Milestone® (7 and 14 fl oz/A) impacted medusahead similarly, butreduced viable seed production of soft brome (chess) and annual ryegrass, both desirable species in California annual grasslands. However, soft brome cover was significantly greater at all Milestone rates compared to non-treated plots because of reduced medusahead competition. On rangeland that is not dominated by desirable annual grasses the impact to grass seed germination would be reduced. On annual grasslands, reseeding may be beneficial if the desirable grass seed bank is depleted due to high density medusahead infestations.
Sidebar: More About Medusahead and Cheatgrass
Medusahead and cheatgrass (downy brome, Bromus tectorum), another invasive annual grass, overlap in distribution and habitat requirements. Both of these invasive grasses can replace more desirable native herbaceous vegetation. Cheatgrass occupies a larger geographical area than medusahead, which extends to drier areas of the semi-arid western United States.
Medusahead ranges from 6 to 24 inches tall and has a seed head with long awns that are stiff and slightly barbed (Figure A). The name medusahead is due to these wiry, twisting awns. The plant was first introduced to the United States in the 1880s near Roseburg, Oregon and is now widespread in the western United States. The invasive grass can reach densities of 1,000 to 2,000 plants per square meter. After seed set, medusahead stems persist as a dense litter layer that prevents germination and survival of native species, ties up nutrients, and contributes to fire danger in summer. Medusahead is not palatable to livestock or native wildlife except early in the growing season because of the high silica content in leaves.
For additional information on identification, distribution, biology and management visit
Rangelands often have a complex of medusahead and broadleaf invasive weeds such as yellow starthistle (Centaurea solstitialis) or other knapweeds (Centaurea sp.). Although the timing for medusahead control is late for optimum yellow starthistle control with Milestone (typical treatment timing for yellow starthistle is November through February), the 3 fl oz/A treatment would likely be successful in controlling seedling and rosette yellow starthistle plants. Concurrent control of medusahead seed production and larger (bolting) yellow starthistle or more difficult to control broadleaf weeds would require a Milestone application rate of 5 to 7 fl oz/A. Application timing for many other invasive broadleaf weeds would be favorable. An increase in desirable grass production may not be realized until the growing season following application because desired grasses germinate in the fall.
1 Authors respectively: University of California (U.C.) Farm Advisor; U.C. Davis Specialist; U.C. Davis Cooperative Extension Weed Specialist; USDA-ARS Sidney MT; U.C. Cooperative Extension Intern
Archer AJ. 2001. Taeniatherum caput-medusae. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/plants/graminoid/taecap/all.html [2017, February 14].
EDDMapS. 2017. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed February 14, 2017.
Kyser GB, DiTomaso JM, Davies KW, Davy JS, Smith BS (2014) Medusahead Management Guide for the Western States. University of California, Weed Research and Information Center, Davis. 68 p. Available at: wric.ucdavis.edu.
Sharp LA, Hironaka M, Tisdale EW (1957) Viability of medusahead seed collected in Idaho. J Range Manage 10:123-126.
Published April 2017; reviewed and updated June 2019
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Active ingredients for products mentioned in this article. Product (active ingredient): Milestone herbicide (aminopyralid).